is there a hp diffrence with tailpipes vs. turndowns

Ron1-
It looks like your Mustang is a track only car, am I right? Well the majority of the people on this site...I would say...have street/strip stangs. Of course if you are on a dyno and want to acheive maximum horsepower, you wouldn't want much back pressure, if any. But that is at wide open throttle. I for one have a Mustang that goes to the track maybe 6-8 times a year & sees a lot of street driving on the weekends. No I don't have dyno sheets to prove that my tailpipes gained me back some low end torque (now remember...that would be at lower rpms...not WOT), nor would I spend the money to do a dyno tune just to see the difference in turn downs or tail pipes. It seems very easy to understand how an exhaust system with some backpressure could benefit a street driven Mustang. Because...if you didn't realize...when you are driving on the street you are not always at WOT. I'm surprised you press this issue so. If you were correct than why isn't every Mustang at the track running open headers (hopefully long-tube ) to gain their combinations full potential??